The present invention relates to a method for treating water which contains nitrogen compounds. More specifically, the present invention relates to a treatment method for water which contains nitrogen compounds, wherein chlorine oxidants or hydrogen peroxide is added. Nitrogen compounds are efficiently removed by oxidative destruction to nitrogen gas. At the same time, chemical oxygen demand (COD) is reduced.
Waste sources of nitrogen compounds include proteins, which are used in livestock feed production industries; nitric acid, which is used in inorganic pigment production; ammonia, nitric acid, sodium nitrate, and sodium nitrite, which are used in surface processing steps in metal products production industries and electronic machinery appliance manufacturing; and the like. When released into water, nitrogen compounds may cause eutrophication. If these nitrogen compounds are released as nitrogen oxides into the atmosphere, they are dangerous to health as primary pollutants. Furthermore, nitrogen oxides participate in a photochemical reaction and become one of the components of smog, a secondary combined pollution phenomenon. As a result, water which contains nitrogen compounds must be processed for eutrophication. Conventional treatment methods for nitrogen compounds in waste water include biological treatment methods, nitration nitrogen removal methods, breakpoint chlorination methods, ion exchange resin methods, electrodialysis methods, and ammonia stripping methods.
When water containing nitrogen is biologically processed, the contact time required is long because the reaction time is relatively slow. A large-volume biological reaction container becomes necessary, and there is an additional problem of a large amount of excess sludge being generated. Furthermore, although it is possible to process a wide variety of organic nitrogen compounds with biological treatment methods, even minor changes in the configuration of organic nitrogen compounds can cause problems when biological treatment methods are employed.
It is known to process waste water which contains hydrazine by an oxidizing treatment method, wherein the waste water is aerated and oxidized in the presence of copper ion. However, because the reaction speed is slow, about a full day is required for treatment.
In waste water which contains monoethanolamine, a breakdown method which employs a catalyzed reaction is known. However, this method has not been widely adopted in the art.
As shown in the equations (I) and (II) below, ammonia is generated through hydrolysis when an aqueous urea solution is heated. When urea reacts with nitrous acid, the urea is converted to dinitrogen. EQU CO(NH.sub.2).sub.2 +H.sub.2 O+heatCO.sub.2 +2NH.sub.3 (I) EQU CO(NH.sub.2).sub.2 +2HNO.sub.2 2N.sub.2 +CO.sub.2 +3H.sub.2 O(II)
Hydrazine dissociates when heated to approximately 180 degrees in the presence of air, Generating ammonia and dinitrogen. Also, it is known that ammonia is generated by the heating and subsequent dissociation of monoethanolamine.
However, methods which apply these decomposition reactions of nitrogen compounds to the treatment of waste water are not known. In the reaction between urea and nitrous acid, a large amount of nitrous acid is required if there is a high concentration of urea. This limits the economical application of this method in the treatment of waste water. In waste water which contains a high concentration of nitrogen compounds, it is difficult to achieve an adequate quality of processed water by merely heat treatment. It is possible to process waste water which contains urea by a wet catalytic oxidation method under high temperature/high pressure conditions (such as 200-300.degree. C., 20-100 kg/cm.sup.2 G). However, this approach is uneconomical, and the conditions are difficult to maintain. Furthermore, even if the nitrogen compounds are completely converted to ammonia, the nitrogen content in the waste water is not reduced. Therefore, further treatment is necessary.
One known method for the physical and chemical treatment method of ammonia is the breakpoint method. The breakpoint method is a method wherein chlorine or sodium hypochlorite is added to waste water which contains ammonia, and the ammonia is removed by oxidizing breakdown into dinitrogen gas. The reactions of the oxidation breakdown treatment of ammonia using the breakpoint method are given below in equations (III) and (IV). EQU 2NH.sub.3 +3Cl.sub.2 N.sub.2 +6HCl (III) EQU 2NH.sub.3 +3NaOClN.sub.2 +3NaCl+3H.sub.2 O (IV)
The characteristic of the oxidizing breakdown of ammonia by the breakpoint method is that more than the stoichiometrically required amount of chlorine or sodium hypochlorite must be added. Normally, this treatment is conducted under normal temperature and pressure conditions, but the reaction mixture must be left standing for the amount of time required for the reaction to proceed to completion. As a result, in order to make the device in which this method is performed more compact, there has been a demand for new technologies, such as, for example, a reaction accelerating medium. In addition, treatment to remove residual chlorine is required when this method is employed, because excess chorine or sodium hypochlorite remain in the processed water.
Japanese Laid Open Publication No. 5-269475 discloses a method for the the oxidizing breakdown of ammonia by hydrolysis, wherein an ammonia-containing solution is processed under heated conditions to accelerate the reaction. However, if large volumes of waste water which contains ammonia are to be heated, problems of excessive energy usage arise.